1. Field of the invention
This invention relates to a toner for rendering an electrostatic latent image visible as in electrophotography, an image forming method making use of the toner, and an apparatus unit having the toner.
2. Related Background Art
In recent years, in image forming methods employing electrophotographic techniques as in copying machines and laser printers, methods are prevailing in which photosensitive members are electrostatically charged with a contact charging member without using any corona charging assembly which may generate ozone, taking environmental problems into consideration. Under such circumstances, especially in an environment of high temperature and high humidity (humidness), a phenomenon in which fine particles are brought into pressure contact with the surface of a photosensitive member by the charging member to adhere to that surface (hereinafter "drum melt-adhesion") has come to occur actually. Also, as to the fine particles as a substance causative of such a phenomenon, analytical means have ascertained that silica fine powder used as a fluidity improver of toners is one of such fine particles.
Such a problem is known to be solved to a certain extent by using a hydrophobic fine silica powder having been treated with a coupling agent comprising hexamethyldisilazane (HMDS) followed by treatment with an oil as disclosed in Japanese Patent Application Laid-open No. 63-139370 (corr. to U.S. Pat. No. 4,868,084), or using a hydrophobic fine silica powder disclosed in Japanese Patent Application Laid-open No. 5-80584. However, as a technical trend in recent years, toners are being made to have much smaller particle diameters because of demands for higher image quality, and the above phenomenon has come to occur still more. Thus, under existing circumstances, it is difficult to solve the problem completely by the use of the above treated hydrophobic fine silica powder.
In addition, a phenomenon in which a low electrical resistance substance formed from paper dust, ozone and so forth generated during printing greatly damages electrostatic latent images formed on the surface of a photosensitive member (hereinafter "smeared images") tends to occur in the case when toners are used in an environment of high temperature and high humidity. As a means for preventing such smeared images, Japanese Patent Application Laid-open No. 60-32060 discloses a toner incorporated with an inorganic fine powder having two types of BET specific surface area. However, studies made by the present inventor have revealed that any satisfactory effect of preventing smeared images can not be attained when, e.g., the above hydrophobic fine silica powder having been treated with a coupling agent comprising HMDS followed by treatment with an oil as disclosed in Japanese Patent Application Laid-open No. 63-139370 is used in the toner as one of external additives in order to prevent the drum melt-adhesion stated above, thus the effect of preventing drum melt-adhesion and the effect of preventing smeared images can not simultaneously be attained.
As another problem, the toner having a high effect of preventing smeared images as stated above can be said to be a toner that tends to abrade photosensitive drums. Use of such a toner may cause a problem of the shortening of drum service life. In such a case, under existing circumstances, a problem may further arise such that the drum surface roughs to cause a decrease in transfer efficiency, which further causes faulty cleaning of the drum and contamination of the charging roller.